GOTRACEBACK=wer is a new traceback level that acts as "crash" and
also enables WER. The same effect can be achieved using
debug.SetTraceback("wer").
The Go runtime currently crashes using exit(2), which bypasses WER
even if it is enabled. To best way to trigger WER is calling
RaiseFailFastException [1] instead, which internally launches the
WER machinery.
This CL also changes how GOTRACEBACK=crash crashes, so both "wer" and
"crash" crash using RaiseFailFastException, which simplifies the
implementation and resolves a longstanding TODO.